http://www.dslzoneuk.net/isp_ratings.php#
good site to rate UK ISP's
I'm with ADSL24 (an entanet reseller) 'home 30' package
http://adsl24.co.uk/broadband_home.php after moving from pipex who were bloody awful by comparison. Currently if I have any small issues then I use the isp's forum and the md and his underlings get right on the problem, they're very honest and approachable too. There are cheaper services, such as those with SKY or Virgin that offer phone tv and all of that nonsense, but you get what you pay for and I really hate the 'buy cheap, buy twice' consequenses of that kind of thinking.
It varies a little, dependent on network traffic, but these guys don't port block or traffic shape to any serious detriment to my connection. Any other shortfall to speed is purely down to the condition of the network itself being able to handle a stable connectin to and from my local exchange.